Q u o t e:We will be introducing a new, hard cap of 600 members in a single guild for Cataclysm. This function will go live with patch 4.0.1 and is already live on the beta and PTR's.

As most of you already know, we have supported a soft cap of 500 members in a guild since World of Warcraft launched. We have allowed guilds to exceed the 500 limit up until now since being in a guild really just amounted to ranks and chat channels. With the advent of the new guild system in Cataclysm we are tracking many more things on each individual player in a guild and in order to support that, we need to limit the amount of members to a reasonable level.

The new cap of 600 members is fully supported in the new guild system and that means that everyone will be visible in the ui and able to contribute to all guild functions like experience and reputation gain. We have pulled a large number of statistics to get to the 600 member cap for guilds and we are happy to say that this value covers more than 99.9% of all the active guilds in World of Warcraft.

The small number of guilds that are over the 600 person cap will be able to keep their guilds intact (and fully supported in the new guild system), but they will not be able to add new members until they fall below the 600 member cap.

tl;dr - hope you don't have more than 600 active members in your guild.

And yeah, "members" = "characters". That person with a main and a couple of alts in-guild? Better boot the alts. Oh wait...that's most people in a guild. Better boot all the alts. Oh, and there will be even more alts come Cataclysm.

Ah well, it's not like there's a number of large RP or GLBT guilds on various servers that are going to get hit extra hard by this or anything.

Q u o t e:When I was on the PTR earlier I joined a guild that had a 1499 member cap that they were trying to reach.
Is this going to change then or will there be something that GMs can buy to increase the member cap of their guilds?

For now there are no plans to implement options that will allow a guild master to extend the hard cap of maximum 600 guild members. We never know what changes and features the future might bring to the game, but it should be safe to expect that this cap will not be changed at any time soon. We are of course not interested in splitting guilds, so those extremely few guilds that currently have more than 600 members will remain intact once patch 4.0.1 is released, but they will not be able to recruit again until they are fewer than 600 members.

We recently said that we had no plans to allow a guild master to extend the hard cap of maximum 600 guild members, which is still true, we do not have plans to allow guild masters to extend the hard cap; however we have made a re-evaluation of what we believe the realms are capable of supporting, and we have decided that we will be enforcing a guild member hard cap of 1000 members instead of the previously announced 600 members..

This means that once patch 4.0.1 is released, guilds that currently has more than 1000 members will no longer be able to invite new members again until they are fewer than 1000 members.

The following information, which was posted recently by Bashiok, explains why we are creating such a hard cap on the number members you can invite to your guild, and it also clarifies a few things about guild levelling and guild management in Cataclysm:

This new guild cap is being enforced for several reasons, but they all factor into a need to control guild sizes in light of the new guild systems, including guild levelling and achievements. Previously, adding players to a guild was as simple as adding people to a chat channel. While approximately 500 members were visible in the UI, there was no real need to limit guild size. That is no longer the case.

Guild levelling in Cataclysm features unified progression powered by a series of complex systems that track the contributions of all guild members. The larger the guild, the bigger the impact on these systems. We found it necessary to determine a maximum guild size to ensure continued performance now and into the future.

We want to reassure the very few guilds that will be affected by this hard cap, that they have plenty of time to plan and rearrange their guilds as they see fit. While this hard cap will go into effect with patch 4.0.1, guilds should feel safe rearranging members as necessary up to the release of Cataclysm on December 7 when the guild levelling system goes into effect.

There will be no impact on guild levelling progression until then. If a guild were to disband after Cataclysm, or a group of people were to leave and start a new guild, they would lose the time and experience they previously contributed and potentially extend the amount of time it would take to regain the new guild perks.

While some players have used options including mods and custom chat channels to support large player and guild alliances that number multiple thousands, groups of that size aren't ideally suited to our design philosophy. As always, we are continually looking into adding new features to help facilitate guild management, scheduling, and player communication.